Aluminum and aluminum alloys are used for forming various electrical connections or wiring in electronic devices such as in integrated circuit structures. The aluminum or aluminum alloys are used for forming the electrical connections between active and/or passive devices of the integrated circuit structure. It has been the practice to use aluminum or an alloy electrically connected to an underlying substrate such as silicon. While the aluminum and silicon are electrically connected together, it has become the practice to use intermediate electrically conductive layers interposed between the silicon and aluminum to provide better electrical connection to the silicon, and to provide a physical (metallurgical) barrier between the silicon and aluminum. This is for the purpose of preventing electromigration and spiking of the aluminum into the silicon. Migration of aluminum atoms into the underlying silicon can interfere with the performance and reliability of the resulting integrated circuit structure.
In addition to electromigration, the problem of hillock growth also occurs. These problems are especially pronounced at the submicron level. As the demand increases for scaling down the dimensions of the interconnection lines and for increasing the current density, overcoming, or at least minimizing, electromigration and hillock growth is essential.
In an attempt to overcome the problems experienced with pure aluminum, aluminum has been alloyed with for instance copper. However, relatively high percentage aluminum-copper (&gt;2%) is known to be difficult to dry etch and corrodes relatively easily.
In an effort to improve on the use of aluminum-copper as the interconnection metallurgy, aluminum-copper has been taught to be layered with a refractory metal such as in U.S. Pat. No. 4,017,890. This patent suggests a method and resulting structure for forming narrow intermetallic strips which carry high currents on bodies such as semiconductors and integrated circuits, wherein the conductive strip includes aluminum or aluminum-copper with at least one transition metal. While the aluminum-copper and transition metal structure improves the electromigration problems associated with aluminum-copper, the problems of etching and corrosion, as well as, the complete elimination of hillocks have not been solved.
With respect to hillocks, such result from the large differences between the thermal expansion coefficients of the metal interconnect lines and the substrate. To eliminate and minimize hillock formation, it has been suggested to use a multi-layered structure instead of a single layer of the interconnect metallurgy. An effective reduction in hillock formation has been achieved by using a multi-layered structure of aluminum or aluminum intermetallic with a layer of a refractory metal. A typical interconnect metallurgy structure includes a layered structure of aluminum silicon compound onto which there has been deposited a layer of a refractory metal, such as, titanium (see "Homogenous and Layered Films of Aluminum/ Silicon with Titanium for Multi-Level Interconnects", 1988, IEEE, V-MIC Conference, Jun. 25-26, 1985).
There have also been refinements to this layered metal structure to provide a lower resistivity, hillock-free, interconnect metallurgy. These refinements include incorporating a barrier metal of, for example, titanium tungsten or titanium nitride under the aluminum silicon to prevent contact spiking and prevent forming ternary compounds in the aluminum silicon alloy (see "Multi-Layered Interconnections for VLSI", MRS Symposia Proceedings, Fall 1987). Moreover, there have been other proposed device interconnect structures to reduce resistivity and provide a more planar and defect free interconnect structure. For instance, IBM Technical Disclosure Bulletin, Vol. 21, April 11, 1979, pp. 4527-4528, teaches the enhancement of the metallurgy for the interconnection due to sputtered deposition. Furthermore, the feature of using a capping layer to improve performance has been disclosed in IBM TDB, Vol. 17, No. 1A, 1984 and TDB, Vol. 21, July 2, 1978.
In addition, U.S. Pat. No. 5,071,714 discloses a structure that includes a low weight copper content aluminum-copper conductor exhibiting superior electromigration characteristics along with being hillock free, dry etchable and corrosion resistant. Moreover, the structures disclosed therein are of relatively low resistivity.
However, there still remains room for improvement of the electromigration characteristics. Accordingly, an objective of the present invention is to provide a structure that exhibits enhanced electromigration performance along with being hillock free and without a concomitant loss in the dry etchability characteristic and corrosion resistant characteristic of the structure.